Project Summary/Abstract Panic disorder (PD) and post-traumatic stress disorder (PTSD) are prevalent, highly comorbid and debilitating psychiatric disorders. Comorbidity is associated with worse patient outcomes, yet little is known regarding the pathophysiology of these conditions. Emerging evidence suggests shared vulnerability factors and underlying mechanisms. One common predictor of vulnerability may be CO2 sensitivity. Low dose CO2 inhalation triggers acidosis and represents a homeostatic stressor which evokes fear. In PD patients, CO2 inhalation reliably induces panic attacks. In veterans, pre-deployment CO2 sensitivity predicted trauma-induced PTSD symptoms. Thus, sensitivity to this homeostatic stressor may predict vulnerability to develop PTSD in response to later trauma. The goal of this project is to investigate the underlying mechanisms and neurocircuitry contributing to shared vulnerabilities and comorbidity of PD and PTSD. Our lab recently found long-term deficits on fear extinction, a common symptom in PTSD patients, in CO2 exposed mice. Individual variability in CO2 responsivity correlated with later extinction deficits supporting a predictive role for CO2 sensitivity on long-term fear outcomes. Our lab previously identified the subfornical organ (SFO) as a key site regulating CO2-evoked fear responses via activation of acid-sensing receptor T-cell Death-associated gene 8 (TDAG8). Genetic deletion of TDAG8 reduced CO2-evoked freezing. TDAG8 expression correlated with PD severity in humans and CO2-evoked freezing in mice. Thus, TDAG8 expression within SFO may regulate sensitivity to CO2 and promote vulnerability to fear extinction deficits. The SFO projects to brain regions associated with fear and anxiety, such as the infralimbic cortex (IL). IL activation was significantly reduced both immediately following CO2 and in mice showing delayed fear extinction deficits and additionally, correlated with fear behaviors. Together, these data suggest that IL activity may mediate the effects of CO2 sensitivity on fear extinction deficits. The objective of this F32 proposal is to elucidate the circuitry and molecular mechanisms by which CO2 sensitivity (homeostatic threat) results in long-term fear extinction deficits (external threat). Direct SFO to IL innervation and the role of SFO TDAG8 in CO2-evoked fear suggest engagement of SFO-IL circuits and upstream SFO TDAG8 chemosensory mechanisms in long term fear extinction deficits evoked by homeostatic threat, CO2. Aim 1 will test the hypothesis that IL hypoactivation via SFO afferents is necessary and sufficient for CO2-evoked long-term deficits on fear extinction. Aim 2 will test the hypothesis that SFO TDAG8 expression regulates CO2 sensitivity and fear extinction deficits through dysregulating fear and homeostatic regulatory brain regions. Results from these studies will identify novel neural circuits regulating fear outcomes and identify predictive markers relevant to PD and PTSD. This will ultimately aid in the development of novel and tailored treatments to better treat or prevent PD and PTSD.